that looks like wood to me :O
Yup, it's a wooden template for routing the water passage and o-ring groove in the acrylic blocks. A guide ring on the baseplate of the router runs along the inside of the template. In the picture, the top piece of acrylic is attached under the template on the left.
Actually, considering where the pipes are mounted they may actually get cooler when a fire is going.
That's what I think, too. I haven't had a fire yet this year, but when one is going there is a lot of air moving through the fireplace. If the temps get too high with a fire going, I'll turn the computer off. Maybe read a book